Carbon fiber is characterized in that, as compared with other fibers, it has a high strength and a high elastic modulus and is light. Therefore, carbon fiber is in wide use for production of composite material products (molded articles) used in various fields such as aviation and space field, sports field and the like. Carbon fiber is being used as a reinforcing material for composite material products containing a thermosetting resin or a thermoplastic resin as the matrix resin. As the matrix resin, a thermosetting resin is often used for its easy moldability and handling.
As the method for molding a composite material containing a thermosetting resin as the matrix resin, there is a method which comprises impregnating a resin into a fiber (a reinforcing material) to make the resin-impregnated fiber into a sheet-shaped prepreg (an intermediate material), and subjecting the prepreg to molding. As other molding methods, there are a pultrusion method, a resin transfer molding (RTM) method, a filament winding (FW) method, a sheet molding compound (SMC) method, a bulk molding compound (BMC) method, a hand lay-up method, etc.
Of these methods, the pultrusion method is suited for production of a long product having a given sectional shape and moreover has an advantage of allowing low-cost molding. As the matrix resin used in this molding method, vinyl ester resins containing substantially or completely no epoxy group have been used in many cases.
The wettability between carbon fiber and vinyl ester resin is poor. Therefore, in composite materials containing a vinyl ester resin as the matrix resin, glass fiber is used mainly as a reinforcing material for the composite material. When a composite material used in applications requiring a strength is produced using a glass fiber, as compared with when there is produced a carbon fiber-reinforced resin composite material containing an epoxy resin as the matrix resin, the amount of glass fiber used need be increased. Therefore, when a composite material of particular strength is produced using a glass fiber as the reinforcing material, as compared with when a carbon fiber is used as the reinforcing material, the composite material produced has drawbacks in that it has an increased sectional area and an increased mass.
As the technique for improving the wettability between carbon fiber and unsaturated matrix resin (e.g. vinyl ester resin) to obtain a high-strength composite material, there is a method of adhering a vinyl ester resin to a carbon fiber (JP-B-1987-18671, Claim 1). In Claim 1 of JP-A-1988-50573 is disclosed a sizing agent for carbon fiber, containing an unsaturated urethane compound as the main component. In Claims 1 to 3 of JP-A-1999-93078 is described a carbon fiber impregnated with a sizing agent having a terminal unsaturated group (e.g. vinyl group) and a polar group (e.g. amide group). In Claim 1 of JP-A-1988-105178 is proposed a method of coating, to a carbon fiber, an ester compound having unsaturated bonds at the two terminals of the molecule, synthesized from a glycidyl ester of an unsaturated monocarboxylic acid and a diol.
In the above method of coating an unsaturated compound to a carbon fiber, described in the above literature, the unsaturated compound coated to the carbon fiber surface and the vinyl ester resin give rise to chemical bonding mainly due to the thermal polymerization taking place between the respective unsaturated groups. However, the bonding between the carbon fiber surface and the compound coated to the carbon fiber surface is adsorption and accordingly is weak. Consequently, peeling tends to occur at the interface between the carbon fiber surface and the vinyl ester resin. As a result, a composite material produced by pultrusion using this method shows no expected high strength in many cases.
In JP-B-1993-40768 is described a curable composition for production of fiber-reinforced resin, which comprises:
an epoxy group-containing acrylate resin containing, in the molecule, an unsaturated ester group in an amount of 0.2 to 0.7 equivalent per 1 equivalent of epoxy group,
a radical-polymerizable monomer,
an organic peroxide,
a curing agent for epoxy resin, and
a carbon fiber.
However, since this composition is insufficient in impregnation rate of matrix resin into reinforcing fiber, the composition has a problem in workability in production of molded article.